Polyester-based resins, such as those formed from saturated and unsaturated polyesters, are utilized in a wide variety of end-use applications. The resins can be used, for example, in conjunction with other types of materials to form a composite. As an example, a prepreg is a composite consisting of fiber reinforcements placed within the resin, which is typically molded with pressure or vacuum to provide a variety of molded articles. Prepregs have applications in aerospace, transportation, appliances, sanitary ware, and the like. Polyester resins can also be diluted with organic solvents such as styrene to form laminating resins or gel coats. Typically, such resins are coated onto components which are used as panels forming the exterior bodies of vehicles, marine vessels, aircraft, and the like.
Traditional processes of forming saturated and unsaturated polyesters have involved the thermal dehydration of polyfunctional organic acids and alcohols, more particularly, dicarboxylic acids and diols. These processes, however, are disadvantageous in that long reaction times are involved, along with extreme processing conditions with respect to temperature and pressure. For example, such reactions occur at temperatures from 200.degree. C. to 260.degree. C. and pressures from 10 to 40 psi and can take from 20 to 40 hours.
Other efforts have focused on producing polyesters by reacting a carboxylic acid or anhydride with an organic carbonate, such as ethylene or propylene carbonate, in the presence of a catalyst. For example, U.S. Pat. No. 3,549,692 to Bockmann et al. proposes the reaction between an anhydride of a carboxylic acid and a cyclic carbonate of an alkylene glycol in the presence of imidazole catalysts. U.S. Pat. No. 4,613,678 to Swart proposes the esterification of aromatic dicarboxylic acids with cyclic alkylene carbonates in the presence of pyridine and pyridine-substituted catalysts. The products formed in these reactions are used in the preparation of polyesters.
The above catalysts, however, suffer from drawbacks. Specifically, the catalysts cause the esterification products to exhibit a high degree of undesired coloration which is aesthetically unpleasing. The products also are capable of forming highly-colored complexes when mixed with carboxylic acid and anhydride monomer in the preparation of saturated and unsaturated polyesters.
In response to the above shortcomings, U.S. Pat. No. 5,714,568 to Nava proposes preparing an esterification product by reacting a polyfunctional organic acid or anhydride, a cyclic organic carbonate, and a catalyst that may be an alkali metal, an alcohol-derived salt of an alkali metal, or mixtures thereof to form the esterification product. Although the '568 Nava patent potentially addresses the above-mentioned problems in the art, there remains a need to provide processes for preparing esterification products which exhibit improved reaction times relative to prior art processes.
In view of the above, it is an object of the present invention to provide processes for forming esterification products that are faster in comparison to conventional processes.